1. Field of the Invention.
The present invention relates to arc coating of metal targets with alloy hard surface coatings.
2. Description of the Prior Art.
Arc coating films onto objects in which a metal target serves as a source for the coating flux applied in a vacuum ambient atmosphere is known. By reacting the flux derived from a metal cathode with a reactive gas such as nitrogen, titanium nitride coatings are now routinely produced using arc coating techniques. These physical vapor deposition techniques produce coatings that offer performance advantages over other surface treatments.
Three physical vapor deposition variations are currently envisioned for wear coating applications, including electron beam evaporation, sputtering, and presently for nonalloy techniques, the cathodic arc. The cathodic arc depositions, the newest of the three, is a high throughput production process that employs multiple arc evaporators to vaporize the coating material. This is presently done with commercial equipment such as that sold by a number of manufacturers, but problems occur in trying to produce coating alloyed from a composite cathode, because of the lack of the ability to steer the arc properly. The process does not permit forming alloy coatings with composite targets.
U.S. Pat. No. 3,625,848 to Snaper and U.S. Pat. No. 3,793,179 to Sablev et al. each disclose apparatus usable for metal coating of parts, but they do not teach precisely controlled arc steering, nor alloyed compound deposition.
To produce films of alloyed materials by vacuum deposition, more than one source of coating flux may be used. This can be done, in principle, with electron beam coating sources, sputtering sources and hollow cathode discharge coating sources. However, precise control of the composition of the film deposited requires precise and proportional control of each coating source. To overcome this problem, alloyed source materials may be used. But differential sputtering and differential fractionation of the alloy during the film deposition process usually leads to the formation of a film with a composition which differs from that of the source material. This problem is partly overcome or compensated by using a source material differing in composition from that of the film desired. The needed composition adjustment is found by trial and error. Even this technique requires exact film deposition condition reproducibility to assure film composition reproducibility.
When arc coating techniques are used to deposit alloy films, composition reproducibility is a much more difficult problem than with vacuum deposition. Arc sources, as taught by the Sablev et al. and Snaper patents mentioned above, produce metal vapor for film deposition and metal droplets which are commonly known as macro-particles. Proportion of the coating material released in the vapor form and in the droplet form for film deposition varies from material to material and with operating conditions. Reproducible arc coating of alloyed material is therefore very difficult.
By acquiring control over the motion of the arc on the cathode, that is the target from which the coating flux is derived, the steered arc coating process allows the precise control over the composition of the film deposited. Control over composition is obtained by controlling the time the arc spends in the different regions of a composite target and by eliminating the formation of macro-particles. The time the arc spends on each part of a composite target is varied by changing the arc velocity on the cathode surface. Therefore, a reproducible composition of the alloyed film is obtained with steered arc coating when a composite target made of more than one material is used and the composition can be changed by changing the arc velocity.